Slanted nanorod structures are widely used in a variety of fields, including the fields of not only semiconductor-based microelectromechanical systems (MEMS) but also photonic crystal devices, display devices, etc.
Copper having excellent electrical conductivity is used in a variety of structures depending on the purposes of the application thereof. In recent years, research into the fabrication of micro-devices using slanted copper nanorod structures has been actively carried out. Slanted copper nanorods can be used as gas sensors, and electrode materials for secondary batteries and supercapacitors.
A technology related to a typical method for fabricating such slanted copper nanorods is disclosed in Korean Patent No. 0281241, and a technology for patterning a workpiece by focusing ions, extracted from plasma, on the workpiece is disclosed in Korean Patent No. 0364207.
Hereinafter, a method of performing plasma etching after changing the grid plane of the top of a Faraday cage and a focused ion beam system and method, disclosed in Korean Patent Nos. 0281241 (hereinafter referred to as “prior art 1”) and 0364207 (hereinafter referred to as “prior art 2”), will be described in brief.
FIG. 1 is a cross-sectional view illustrating an etching system that is implemented by bringing a Faraday cage having a grid plane slanted with respect to a substrate into electrical contact with the cathode of a TCP plasma etching reactor, according to prior art 1.
Referring to FIG. 1, an etching method using a Faraday cage 14 enables a substrate to be patterned in a relatively simple manner in a high-density plasma atmosphere, and thus is frequently used for the fabrication of slanted nanorods. In this case, the Faraday cage 14 refers to a closed box made of a conductor. When the Faraday cage 14 is placed in plasma, a sheath is formed on the outer surface of the cage so that an electric field in the cage is maintained constant. In this case, when the top of the cage is replaced with a fine grid, the sheath is formed along the surface of the grid. Accordingly, ions, which are accelerated in the sheath formed horizontally on the surface of the grid, are incident into the cage and then arrive at the substrate while maintaining the incident directionality thereof. Therefore, the ion incident angle can be adjusted as desired by varying the slope of a workpiece holder. The use of this Faraday cage 14 has an advantage in that a slanted etched structure can be fabricated in one step. However, the slanted etching method using the Faraday cage 14 according to prior art 1 has a disadvantage in that it may be applied only when a material that can be etched by plasma is used. For example, in the case in which a material to be etched is copper as shown in FIG. 2, plasma etching cannot be applied thereto. Due to this problem, there has been no method for fabricating slanted copper nanostructures using plasma, and no particular solution thereto has been found to date.
FIG. 3 is a cross-sectional view illustrating an example of a focused ion beam system according to prior art 2. Referring to FIG. 3, focused ion beam etching (FIBE) that is performed by the focused ion beam system of prior art 2 is a method that includes extracting ions, formed in plasma, by acceleration, and etching a specific portion of a substrate by focusing the extracted ions thereon. This method enables the independent control of parameters contributing to etching, including ion beam directions, ionic fluxes, energy and the like, and thus has been widely used for the fabrication of slanted nanorod structures. In addition, this method has an advantage in that since it etches the workpiece using the physical energy of ions, it can form nanorod structures regardless of the type of workpiece.
However, to perform ion beam etching according to prior art 2, a complex ion source and a system for focusing extracted ions on a workpiece are required, and thus the etching process is highly expensive. Furthermore, since the portion to be etched corresponds to only the diameter of an ion beam, only a very restricted portion is etched. Moreover, the etching speed of ion beam etching is very low because physical sputtering acts as a major mechanism. To overcome the problem of the low etching speed of ion beam etching, techniques have been introduced, including reactive ion beam etching (RIBE) in which reactive gas is incorporated into an ion source, or chemically assisted ion beam etching (CAIBE) in which reactive gas is additionally injected immediately before an ion beam reaches a substrate. However, since the content of reactive radicals that participate in etching in RIBE or CAIBE is much lower than that in general plasma etching, the etching speed of RIBE or CAIBE is as low as 1/10 or less of the etching speed of general plasma etching, and thus the process yield of RIBE or CAIBE is very low. Therefore, RIBE or CAIBE suffers from difficulty fabricating a large-area workpiece.
As a result, in the prior arts, a technology for controlling the angle and aspect ratio of slanted patterns is essential to form slanted copper nanorod structures. However, a technology capable of fabricating slanted nanorod structures in large amounts has not yet been developed. Accordingly, ion beam etching techniques that pattern a workpiece simply using the physical energy of particles have been chiefly used. As described above, the ion beam etching techniques have shortcomings, including ion implantation attributable to the high bombardment energy of ions, the lattice defect of a workpiece, the distortion of an etched shape attributable to the re-deposition of sputtered particles, etc. In addition, these techniques cannot fabricate a large-area workpiece, and thus are not suitable for commercialization.